The present invention relates to an electrophotographic process, and more particularly, to such a process for forming an electrostatic latent image of a high contrast voltage on an electrophotographic photosensitive member.
As is well recognized, an electrophotographic photosensitive member has a basic construction in which a conductive supporter, a photoconductive layer and a transparent insulator layer are sequentially laminated. By way of example, in an electrophotographic process which forms an electrostatic latent image on the photosensitive member, it has been known to employ a process in which after a primary charging, a secondary corona charge is applied by a sinusoidal wave a.c. corona discharging during an imagewise exposure and subsequently an electrostatic latent image is formed by a uniform exposure (Japanese Patent Publication No. Sho 42-23910).
However, the conventional electrophotographic process employs a conventional sinusoidal wave a.c. corona discharging method to provide a secondary corona charge and hence it has a disadvantage in that even if a positive and a negative voltage-time integration value of a sinusoidal wave alternate current applied to a corona charger are identical, the charging inclines toward the negative side because the corona discharging method has the characteristics of a negative corona discharging being more liable to occur than a positive one. Accordingly, for example, when the photosensitive member is a photoconductive layer of the p-type, with the electrophotographic process, a primary charge is provided with negative charge, a secondary charging is applied with a sinusoidal a.c. corona charging method during an imagewise exposure and then a uniform exposure is provided to form an electrostatic latent image. As a result of this process, it is difficult to sufficiently neutralize and remove the negative charge at a bright area of an image and to render the contrast voltage between the bright and dark areas of the image sufficiently high. At this time, a potential level at the bright area indicates a negative value (-100 to -600 V) and when the latent image is developed with a positive charge toner a photographic fog is disadvantageously produced.